1. Field of the Invention
The present invention relates to bone fixation material easily biodegrading and bioabsorbing into the living body organism and methods for preparing the materials. More particularly, the present invention relates to bone fixation materials for bone medical treatment use in plastic surgery, oral surgery, thoracic surgery and in addition animal bone treatment and methods for preparing these materials.
2. Description of the Related Art
Artificial biomaterials having a high bending strength are employed in order to redress bone fractures or support chests for fractured bone treatment in plastic surgery, oral surgery, and thoracic surgery. It is necessary that the bone fixation materials such as plates and screws are applied in the period that the bone is redressing and are removed as soon as possible after healing in order to prevent the bone from weakening.
Nowadays, although the bone fixation materials in clinical use are almost all composed of metals or ceramics, there are problems in which materials cause the bone to degrade, because they have too high of a level modulus, or happen to biodegrade or the metal ions are possibly dissolved into the bone. If materials having similar modulus to the bone, and which biodegrade and bioabsorb into a living organism, are employed in order to redress the bone fracture, various ill effects on the redressing of the bone will disappear because no alien substances take place in the living body over a long time.
According to the above conception and expectations, lactic acid polymers which have the characteristic of biodegrading and bioabsorbing into the living body organism are gradually being employed right now as bone fixation materials in clinical application use. It has been well-known for many years that the lactic acid polymers could be employed as bone fixation materials. For example, Japanese applied patent 59-97654 discloses a method to synthesize a lactic acid polymer, but fails to disclose a lactic acid polymer composite having a high level modulus and high level bending strength.
Japanese patent 3-63901 discloses molding conditions in which a lactic acid polymer composite contains hydroxyapatite and has the characteristic of biodegrading and bioabsorbing into the living body organism improved by uniaxial extension, mechanical properties such as modulus and bending strength useful for bone fixation materials. PCT patent WO/05312 discloses other molding conditions similar to said patent, in which a lactic acid polymer rod was uniaxially extended to a relative high draw ratio (7 to 12 times) at a relatively high temperature and attained a high level bending strength, and a high level modulus by fibrillation. Japan applied patent 1-198553 discloses bone cementing materials, of which lactic acid polymer was composed of a viscosity-average molecular weight of more than 200,000 after molding and uniaxial extension. Furthermore, Japanese applied patent 9-135892 also discloses molding conditions, in which molecules were oriented parallel not to the main, but to the plural axes during molding in order to reduce anisotropy of the bending strengths in a composite.
However, the patents mentioned above only concern initial bending strength and the hydrolyzing durability of bone fixation materials and fail to disclose its bending strength in a living body organism and its durability. There is not enough description concerning biodegradation and bioabsorption into the living body organism after redressing, and especially no description of the method to promote biodegradation and the bioabsorption after the bending strength has left. It takes only about 6 to 8 months to perfectly redress a bone fracture. However, the bending strength of the bone connecting materials characteristically biodegrade and bioabsorb, gradually decreasing after embedding into the body in accordance with the decrease of the molecular weight, and decrease to 0 within 4 months. The molecular weight of the materials reduces from about 200,000 at first to several thousands, to several tens of thousands, while a lot of granular crystallite still remains for 3 to 5 years (J. E. Bergsma et al. Biomaterials, 16(4),267-274,1995). As a result, when the bone connecting materials mentioned above are employed for a material to redress bone fractures, it takes, for example, from 3 to 5 years for the fibrous tissues to perfectly retrieve bone tissue, because the velocity of biodegradation and bioabsorption is too slow. In addition, recreated bones can not invade into and embed with parts of the bone connecting materials even after healing and, therefore, vomicae in the bone remains almost forever. It causes a reduction in mechanical tenacity which bears a load at first. Added to the above problem, many serious late complications can occur after healing (J. E. Bergsma et al. Biomaterials, 16(1), 25-32,1995).